1. Field of the Invention
This invention relates to an integrated circuit device composed of insulated gate field-effect transistors (hereinbelow term "MOSIC") and also to a method of manufacturing the same.
2. Description of the Prior Art
In general, a MOSIC uses aluminum or silicon as the material for the gate electrode. On the other hand, there are integrated circuits (IC) in which, in one internal circuit of an inverter stage, a load MOSFET of the enhancement type is employed as the load of a driving MOSFET operative in the enhancement mode (hereinafter called "E/E type IC") and there is an IC in which a load MOSFET of the depletion type is employed (hereinafter called "E/D type IC"). Among the ICs, the silicon gate MOSIC employing the depletion load has recently been extensively adopted as a device which is of higher performance for various uses and has a higher desnity of integration than the aluminum gate MOSIC. According to experiments by the inventor, the occupying area of an Si gate MOS transistor having self-aligned gate structure is reduced by approximately 20 to 30% as compared with that of an Al gate MOS transistor.
As a result of the investigation of MOSICs commercially available as produced by several manufacturers, however, it has been found that, in a read only memory (hereinbelow abbreviated to "ROM") which occupies a considerable part of a MOSIC chip, the size of the single bit of the ROM is not always remarkably smaller in the conventional Si gate ROM structure than in the Al gate one as indicated in the following Table 1:
Table 1 ______________________________________ Comparisons of ROM bit sizes IC manufacturer Si gate ROM Al gate ROM type of IC ______________________________________ A -- .mu.m.sup.2 336 .mu.m.sup.2 dynamic type B 616 650 static type C 525 -- static type D 412 -- static type E 441 432 static type ______________________________________